Pulse generator

Abstract

A pulse generator comprises a comparator to the inputs of which are connected a capacitor chargeable by a power source and a voltage divider respectively, two transistors being connected to the output of the comparator, one to set the voltage ratio of the voltage divider and one to provide a discharge path for the capacitor and two additional transistors being selectively connected to the comparator output, one providing a different voltage ratio of the voltage divider and the other providing an independent discharge path for the capacitor.

Description

BACKGROUND OF THE INVENTION

The invention relates to a pulse generator with a switchable output frequency and a selectable clock pulse ratio with the different output frequencies from a comparator, a voltage divider being connected to its input as well as a capacitor which is chargeable by the supply voltage, whereby an output signal dependent on the ratio of the input voltages arises at the output of the comparator, as well as having two transistors connected to the output of the comparator, whereby the divider ratio of the voltage divider is adjusted so as to determine the frequency via one transistor, while the other represents a discharge current path for the capacitor.

Pulse generating circuits are already known in which two voltages are compared in a comparator. One of the two input voltages constantly changes until it reaches the value of the other fixed input voltage and thus brings about a change in the output signal from the comparator.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a circuit for a pulse generator with the aid of which pulses with a switchable frequency and with a freely selectable clock pulse ratio may be produced at the output of the comparator.

According to the invention, there is provided a pulse generator comprising a comparator, a voltage divider connected to a first input of said comparator, a capacitor chargeable by a supply voltage and connected to a second input of said comparator, a first transistor connected to the output of said comparator for adjusting the voltage ratio of said voltage divider to determine the output frequency of said output, a second transistor connected to said output for forming a discharge current path for said capacitor, a third transistor for varying the voltage ratio of said voltage divider, a fourth transistor for providing a second discharge current path for said capacitor and switch means for selectively connecting said third and fourth transistors to said output of said comparator.

Further according to the invention, there is provided a pulse generator with a switchable output frequency and a selectable clock pulse ratio for the different output frequencies from a comparator, to the input of which is connected a voltage divider and a capacitor which is chargeable by the supply voltage, whereby an output signal dependent on the ratio of the input voltages arises at the output of the comparator, as well as having two transistors connected at the output of the comparator, whereby the voltage ratio of the voltage divider is adjusted by one transistor so as to determine the frequency, while the other transistor forms a discharge current path for the capacitor, characterized in that at least two further transistors are connectible to the output of the comparator via a switch whereby the divider ratio of the voltage divider may be set by one transistor so as to change the frequency, while the other transistor path represents a further discharge current path for the capacitor which is independent of the first discharge current path.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in greater detail, by way of example, with reference to the drawings, in which:

FIG. 1 is a circuit diagram of one embodiment of the invention;

FIG. 2 is a graph showing the waveform of the voltage across the capacitor in the embodiment of FIG. 1, and

FIG. 3 is a graph showing the waveform of the output voltage of the embodiment of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In a preferred embodiment of the invention, in a pulse generator of the type described at the outset, at least two further transistors are connectible to the output of the comparator via a switch, whereby the divider ratio of the voltage divider may be set in a manner changing the frequency via one transistor, while the other transistor path represents a further discharge current path for the capacitor.

Thus, with a closed switch, the base electrodes of four transistors are connected to the output of the comparator, whereby a decoupling resistor is preferably arranged between the base electrode of each transistor and the connection to the comparator output. The transistors changing the divider ratio of the voltage divider have collector resistances which are connected in parallel with a resistor of the voltage divider via the transistor acting respectively as a switch. The two current branches from the collector emitter path of the transistors and the respective collector resistance are thus connected in parallel with one resistor of the voltage divider.

At least one of the transistors representing the discharge current paths for the capacitor has a collector resistance determining the discharge time constant and thus the clock pulse ratio. Obviously there is also the possibility of connecting a variable or fixed resistor in both current paths. The circuit in accordance with the invention may be used particularly well as a control circuit for the flashers of a motor vehicle. The flash frequency is then changed in a desirable manner by the circuit in accordance with the invention in the case of failure of the load while simultaneously changing the clock pulse ratio. All of the circuit members, with the exception of the capacitor and the charging resistor, may be integrated into a common semiconductor body. The comparator is preferably formed by a differential amplifier of conventional type.

With the pulse generator in accordance with the invention, it is also advantageous if the circuit has a self-starter with defined starting characteristics. Furthermore, the circuit only requires one capacitor. For both frequencies which may be produced at the output of the comparator, the clock pulse ratio may be freely selected independently of the other frequency by dimensioning the divider resistances and the resistances in the discharge current paths.

Referring now to the drawings, FIG. 1 shows the circuit in accordance with the invention. Two current branches are connected in parallel to each other between the two poles of the supply voltage U_S. One current branch contains a voltage divider made up of resistors R₃ and R₆. The other current branch is formed by the series connection of impedances R₁, R₀ and C₀, whereby, at the same time, this is the charging current path for the capacitor C₀. The capacitor voltage U_C forms one input magnitude of the comparator D. The other input magnitude of the comparator is derived at the voltage divider and in fact at the connection between the resistors R₃ and R₆. Two current branches are connected in parallel to the resistor R₆ of the voltage divider and these comprise the collector-emitter path of the transistors T₃ or T₄ respectively and the related collector resistor R₄ or R₅. Two further current branches are connected in parallel with the series connection of resistor R₀ and capacitor C₀. One current branch comprises the collector-emitter path of the transistor T₂, while the other current branch comprises the collector-emitter path of the transistor T₁ and the related collector resistor R₂. The base electrodes of transistors T₁ and T₄ are connected to the output of the comparator via decoupling resistors. The base electrodes of the transistors T₂ and T₃ may also be connected to the output of the comparator D via a switch S and appropriate decoupling resistors.

The circuit described and shown in FIG. 1 operates as follows:

After applying the supply voltage U_S, the capacitor C₀ is charged up via R₁ and R₀. The waveform of the capacitor voltage U_C is shown graphically in FIG. 2. The voltage U₂ across resistor R₆ is determined by the divider ratio between the resistors R₃ and R₆. The voltage U_C and the voltage U₂ form the input magnitudes of the comparator, the comparator preferably comprising a differential amplifier. During the time t₁, in which the capacitor voltage U_C has not yet reached the value U₂, there is a low potential applied to the output U out of the comparator in accordance with FIG. 3. Initially it is assumed that the switch S is closed so that all base electrodes of the transistors T₁ to T₄ are connected to the output of the comparator via decoupling resistors. Since there is a low potential applied to the output of the comparator all of the transistors remain blocked.

The charging time constant of the capacitor is dependent on the resistors R₀ and R₁ and on the capacitance of the capacitor C₀. If R₀ is very much greater than R₁ then the dependence of the charging time constant on resistor R₁ may be neglected. If U_C reaches the value U₂ and thus both input magnitudes at the comparator have the same value then the comparator switches over and in accordance with FIG. 3 there is a high potential at the output of the comparator. As a result of the potential increase at the output of the comparator, all of the transistors T₁ to T₄ are through connected. This means that the resistors R₄ and R₅ are connected in parallel to the resistor R₆ and thus the input potential passed to the comparator and taken off the voltage divider drops to the value U₁. The current conducting saturated transistor T₂ discharges the capacitor C₀ via R₀ until the capacitor voltage reaches the lower threshold value U₁. This discharge time which is determined substantially by the resistance value of R₀ and the capacitance C₀ is designated t₂ in FIG. 2. During this time the high output potential remains at the output of the comparator. The discharge current path also connected up across the transistor T₁ and the resistor R₂ may be neglected when the switch S is closed because of the semiconductor resistance present. If the capacitor voltage U_C has reached the value U₁ then low potential sets in again at the comparator output whereby all the transistors are blocked again and the capacitor C₀ is charged up to the value U₂ again in the time t₃. The time ratio t₃ /t₂ determines the clock pulse ratio of the frequency f₁.

In accordance with FIG. 3 the fairly large frequency f₂ at the output of the comparator D is set by opening the switch S. With an open switch S the voltage value U₂, up to which the capacitor may be charged, remains unchanged since it is predetermined by the divider ratio of resistors R₃ and R₆. If the capacitor voltage U_C reaches the value U₂ then the high potential sets in again at the comparator output as a result of which however only transistors T₄ and T₁ are triggered. As a result, only the resistor R₅ is connected in parallel to the resistor R₆ and the potential U₃ derived at the voltage divider is at the comparator input, said potential U₃ being substantially above the potential U₁. The capacitor C₀ may be discharged therefore only up to the value U₃. The discharge current path of the capacitor C₀ now must pass through the collector resistor R₂ of the transistor T₁ via the resistor R₀ and via the through connected collector-emitter path of this transistor T₁. The collector resistor R₂ permits a maximum theoretical discharge of the capacitor at the value U₄, which is below the value U₃ however and therefore cannot be reached. The resistance values R₀ and R₂ together with the capacitance magnitude determine the discharge time constant t₄ for the capacitor C₀, which is independent of the discharge time constant at the frequency f₁.

The voltage curve at the capacitor C₀ is again shown graphically with the open switch S in FIG. 2. From the ratio between the discharge time t₄ and the charge time t₅ is given the clock pulse ratio of the output pulse of the frequency f₂ in accordance with FIG. 3. Thus it is ensured that the clock pulse ratio of the frequency f₂ is independent of the clock pulse ratio of the frequency f₁.

In the following an example of the circuit constants in accordance with the invention is given which is particularly suitable when using the circuit for a flasher unit of a motor vehicle. The stated frequency f₁ indicates a functioning display with its loads intact, while the frequency f₂ appears if at least one of the loads has failed. As a result of this marked frequency change, for example it is a question of almost three times the frequency, failure of a load is clearly indicated.

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Examples of dimensioning:
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R1 = 5.5k Ohm
Co = 3.3uF
R2 = 6.5k Ohm
Decoupling resistors for
R3 = 4k Ohm
T1 -T4 = 11k Ohm
R4 = 1.6k Ohm
R5 = 24k Ohm
Derived (leakage)) resistor
R6 = 12k Ohm
at S = 51k OHM
US = 10V
t1 = 0.44s
f1 = 1.47Hz
U2 = 7.5V
t2 = 0.33s
f2 = 4.08Hz
U3 = 6.67V
t3 = 0.35s
U4 = 5.32V
t4 = 0.153s
U1 = 2.5V
t5 = 0.092s
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It will be understood that the above description of the present invention is susceptible to various modification changes and adaptations.

Claims

1. A pulse generator with a switchable output frequency and a selectable clock pulse ratio in the different output frequencies from a comparator, to the input of which is connected a voltage divider and a capacitor which is chargeable by the supply voltage, whereby an output signal dependent on the ratio of the input voltages arises at the output of the comparator, as well as having two transistors connected at the output of the comparator, whereby the voltage ratio of the voltage divider is adjusted by one transistor so as to determine the frequency, while the other transistor forms a discharge current path for the capacitor, characterized in that at least two further transistors are connectible to the output of the comparator via a switch whereby the divider ratio of the voltage divider may be set by one transistor so as to change the frequency, while the other transistor path represents a further discharge current path for the capacitor.

2. A pulse generator as defined in claim 1, wherein the base electrodes of the said two transistors are connected to the output of the comparator, while the base electrodes of said additional transistors are connectible to the output of the comparator via a switch.

3. A pulse generator as defined in claim 1, wherein the two transistors changing the divider ratio of said voltage divider have collector resistors and two current branches made up of the collector-emitter path of the said two transistors with their collector resistors are connected in parallel with one resistor of said voltage divider.

4. A pulse generator as defined in claim 1, and comprising a collector resistor for at least one of the transistors forming the discharge current paths for the capacitor determining the discharge time constant and thus the clock pulse ratio.

5. A pulse generator comprising a comparator, a voltage divider connected to a first input of said comparator, a capacitor chargeable by a supply voltage and connected to a second input of said comparator, a first transistor connected to the output of said comparator for adjusting the voltage ratio of said voltage divider to determine the output frequency of said output, a second transistor connected to said output for forming a discharge current path for said capacitor, a third transistor for varying the voltage ratio of voltage divider, a fourth transistor for providing a second discharge current path for said capacitor and switch means for selectively connecting said third and fourth transistors to said output of said comparator.